Certain colloidal clays will swell in water to many times their dry volume, and as such are of utility as gelling or thickening agents for control of the rheological properties of a variety of materials. These naturally occurring clays are generally not compatible with a number of organic-based compositions. Hence, it is necessary to organically modify these clays to make them compatible with organic materials, and such clays are generally referred to as "organophilic" clays or "organo-clays". These clays have a large number of uses for thickening organic compositions such as paints, lubricants, cable filling compositions and the like.
The basic starting material used to make organophilic clay is an exchangeable clay of the smectite group and can include montmorillonite (commonly known and mined as bentonite), hectorite, saponite, attapulgite and sepolite. These clays include exchangeable cationic species such as sodium, potassium or calcium ions on their surface. In the course of manufacturing an organophilic clay, at least a portion of these exchangeable cationic species are substituted by an organic cation such as a quaternary amine, an organophosphorus ion, any other ion of the type known in the art as an oniumion, or the like. The addition of the organic group modifies the clay, making it compatible with organic compositions and in general allowing it to provide rheological modification in select organic fluids.
There is a large body of prior art relating to the manufacture of organophilic clays. For example, U.S. Pat. Nos. 2,531,427 and 2,531,440 both disclose general processes for the manufacture of organically substituted clays. U.S. Pat. No. 5,110,501 discloses a process for preparing organophilic clay in which the starting materials are reacted in a slurry under high shear. U.S. Pat. No. 5,334,241, discloses a low moisture, direct addition process for the fabrication of organophilic clays. U.S. Pat. No. 4,569,923 discloses yet another process for manufacturing organo-clays in which the clay is subjected to high energy pugmilling before reaction with the cation. In addition to processing parameters, the properties of an organo-clay will depend upon the nature of the organic cation used for modification, as well as upon the amount of the cation incorporated into the clay. Specific modifiers are utilized for particular mixtures.
It has been found that the amount of organic cationic material which is incorporated into the clay will depend upon the particular technique used for its incorporation. The clays of interest have a generally lamellar structure, and the structure must be opened to some degree in order to permit the cation exchange reaction to take place. At the same time, care must be taken to avoid destroying the lamellar microstructure of the clay. One group of substitution techniques involves processing the clay in a slurry form, in which instance, solvation relaxes the clay's structure in order to permit penetration of the organic cations. While such techniques are effective, and widely employed, it is desirable to avoid such highly dilute reaction conditions since forming the slurry involves a time and space consuming step. Furthermore, dewatering of the finished product is energy-intensive. For this reason, low moisture, direct addition techniques as described in U.S. Pat. N. 5,334,241, the disclosure of which is incorporated herein by reference, are advantageously employed in the preparation of modified clays.
The present invention recognizes that in a direct addition process such as that disclosed in U.S. Pat. No. 5,334,241, it is desirable to achieve a high degree of substitution in the clay, while minimizing damage to its microstructure. The invention further recognizes that conventionally employed measures of the energy input utilized in the manufacture of an organo-clay, as for example in terms of horsepower hours per ton, are not good indicators of success in the fabrication of a high performance organo clay in a direct addition process. The present invention recognizes that high pressure conditions are needed in order to achieve a high degree of substitution with minimal degradation of the clay structure. It has been found that clay processed under high pressure conditions is superior to an equivalent clay processed under lower pressure conditions, even though the overall energy input thereto is the same, and it is postulated that this is because a low pressure reaction, is not adequate to provide a high degree of clay substitution; furthermore, it has been found that if the low pressure reaction is carried out for a sufficiently long period of time it only serves to degrade the clay structure. Therefore, the present invention recognizes that clays produced under high pressure conditions are superior to those produced under lower pressure conditions, even though the total energy input in the low pressure reaction is the same or greater. It has further been found that multi-stage processing of clay material, wherein reactants are subjected to at least two cycles of high pressure mixing provides an improved product as compared to single stage processing of materials. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.